A known image reading device reads a reference member, which is a basis for brightness of an image reading means, and sets an light-source adjusting value of a light source. The image reading device determines presence or absence of external light on a basis of a value read from the reference member and, upon determination that the external light is present, emits light from the light source using the light-source adjusting value that has been preset during manufacturing or before shipment.
However, in a case of reading a document while changing the brightness of the light source in each reading mode determined under reading conditions such as a resolution and a reading speed, the brightness of the light source differs depending on the reading mode. Because of this, the known image reading device, which determines using the light-source adjusting value that has been preset during manufacturing or before shipment could make erroneous decision on the presence or absence of the external light.
Thus, there is a need for an image reading device that can determine the presence or absence of the external light with higher accuracy and with a better performance even when the brightness of the light source differs depending on the reading mode.